1. Field of the Invention
The present invention generally relates to a magnetic disk unit, and more particularly to a magnetic disk unit in which a magnetic head to be used is selected from a plurality of magnetic heads.
2. Description of Related Art
Recently, in magnetic disk units, to increase the capacity of each magnetic disk, the track pitch which is the distance between adjacent tracks on each magnetic disk has been decreased. The magnetic disk unit has a plurality of magnetic heads mounted at ends of arms of a head actuator so as to face respective magnetic disks. The head actuator drives the magnetic heads so that all the magnetic heads are positioned at a cylinder in the magnetic disks. In this magnetic disk unit, due to temperature variations, each magnetic head may be misaligned with a track, in the cylinder, at which each magnetic head is to be positioned on a corresponding magnetic disk. This is referred to as a thermal off-track error. The smaller the pitch on the magnetic disk, the more greatly the thermal off-track error affects read/write operations in the magnetic disk unit.
During a seek operation of a positioning circuit in accordance with a seek instruction from a host system, the magnetic heads are positioned at a cylinder in the magnetic disks. In a case where the track pitch is large enough to ignore the thermal off-track error of each magnetic head, all the magnetic heads can be positioned at tracks in the same cylinder. In this case, immediately after a magnetic head by which data is to be read or written is switched in accordance with a head switching instruction from the host system, the data can be read from or written in a corresponding track in the cylinder at which the magnetic heads are positioned. The positioning circuit activates a track follow signal (TRK-FLW) indicating the allowance of the read/write operation after the seek operation. In the above case, even if the magnetic head by which the data is to be read or written is switched off, the track follow signal (TRK-FLW) is maintained to be activated. When the track follow signal (TRK-FLW) output from the positioning circuit is activated, the host system can at any time supply a read/write instruction to the magnetic disk unit.
On the other hand, in a case where the track pitch is small so that the thermal off-track error of each magnetic head cannot be ignored, all the magnetic heads are not always positioned at tracks in the same cylinder. Thus, when the magnetic head is switched from a first head to a second head in accordance with the head switching instruction supplied from the host system after the seek operation is completed so that the first head is positioned at a corresponding track in a cylinder, a thermal off-track error of the second head must be compensated for. That is, to compensate for the thermal off-track error of the second head, all the magnetic heads are moved so that the second head is precisely positioned at a corresponding track in the cylinder. In this case, the track follow signal (TRK-FLW) is deactivated until the compensation for the thermal off-track error is completed.
The positioning circuit outputting the track follow signal (TRK-FLW) is generally formed of a digital servo-controller (MPU). The digital servo-controller monitors whether or not the head switching instruction (SW INSTRUCTION) is supplied from the host system at a predetermined intervals (e.g. 10 .mu.sec.) in synchronism with a sampling clock signal as shown in FIG. 1. Thus, the track follow signal (TRK-FLW) is not always inactivated immediately after the head switching instruction is supplied from the host system. When the track follow signal (TRK-FLW) signal is deactivated at a time t1 after a time t0 at which the head switching instruction is supplied from the host system, the read/write operation is allowed between t0 and t1, as shown in FIG. 1, although the compensation of the thermal off-track error is not completed. Thus, in this case, original data may be erased from a track adjacent to a track in which data is to be written by overwriting of data.